The proposed study is on a strain of Amoeba discoides, spontaneously infected with a large number of rod-shaped bacteria (60,000 - 150,000 per amoeba). The infective bacteria were initially harmful to the host amoebae, and were regarded as parasites. However, within 5 years of infection (less than 1000 amoeba generations), the bacteria became not only harmlesss to amoebae, but essential to the survival of hosts. Thus intracellular parasites changed to beneficial symbionts within an observable period of time, the first recorded instance of its kind. The objectives of the project are to study the structural and metabolic relationship between the host cells and the infective organisms to elucidate the basis of their mutual dependence. We shall study the kinetics of infection and the development of amoeba's dependence in detail after infecting normal amoebae experimentally using micromanipulative procedures. At the same time, the infected amoebae, at different stages of infection, will be examined under the electron microscope to study the structural relationship. The infected amoebae and bacteria will also be studied for their mutual metabolic dependence by various biochemical methods, including the use of radioisotopic tracers. The results are expected to contribute considerably to our understanding in several areas of cell biology, such as intracellular symbiosis, origin of eukaryotic cell organelles, and cell variation and evolution.